The 191cm forward returned to the field after the injury but scans revealed a small fracture adjacent to his knee joint.
In positive news for Johnston and the Crows, there is no ligament damage to his knee and the injury won't require surgery.
Johnston, 22, was right in the mix to help cover the loss of Taylor Walker after consistent SANFL form for North Adelaide.
He booted seven goals in one match earlier in the season and played a pivotal role in the Roosters's charge to last year's SANFL preliminary final.
Crows football manager Phil Harper said it was an unlucky setback for the talented forward, who North Adelaide coach and Port Adelaide legend Josh Francou described as the best kick for goal he had ever seen.
"It’s really disappointing for Lewis because he’s had a good start to the year in the SANFL and was pressing for senior selection," Harper said.
"We’ll support Lewis in his rehabilitation and look forward to him getting back and playing good footy again later in the season."
The latest setback for the club's forward line led Crows coach Brenton Sanderson to admit he might have to look to defence to find the club's goal kickers.
Defenders Matthew Jaensch and Andy Otten both booted three goals against Carlton in round five while Graham Johncock is currently the SANFL's leading goalkicker after impressing up forward with Port Adelaide.
"We started Jaensch there tonight because he showed us something last week, Andy Otten was fantastic again tonight down back but maybe he's an option for us going forward," Sanderson said after Saturday night's 11-point loss to Hawthorn.
"We might have to make some adjustments."
Harry Thring is a reporter for AFL Media. Follow him on Twitter: @AFL_Harry.
